This invention relates to a copolymer which can be used as a thickening agent for aqueous preparations which have a pH above 5.5, to a method for its preparation and to its use as a thickening agent.
Polymers which thicken aqueous preparations as a function of the pH are known in the art.
In EP-A1 0 216 479, copolymers are described which are water soluble or water swellable in alkaline or acidic media, but which are insoluble or not swellable by water at a pH of 7. Such copolymers are prepared by the emulsion polymerization of:
a) 0 to 90% by weight of ethylenically unsaturated, ionic monomers, PA1 b) 0 to 90% by weight of ethylenically unsaturated, mainly nonionic monomers, PA1 c) 0.5 to 100% by weight of a ether of the formula EQU CH.sub.2 =CR'CH.sub.2 --O--A.sub.m B.sub.n A.sub.p --R PA1 wherein R' is H or methyl, A is a propyleneoxy or butyleneoxy group, B is an ethyleneoxy group, n is equal to 0 or a whole number, usually 1 to 100, m and p are each 0 or a whole number less than n and R is a hydrophobic group with at least 8 carbon atoms and PA1 d) 0 to 5% by weight of a cross-linking monomer, preferably a copolymerizable, polyethylenically unsaturated monomer. PA1 10 to 50% by weight of acrylic or methacrylic acid, PA1 25 to 75% by weight of a nonionic vinyl monomer and PA1 2 to 50% by weight of a macromonomer of the general formula EQU CH.sub.2 =CR--CH.sub.2 --O--(C.sub.n H.sub.2n O).sub.a (C.sub.m H.sub.2m O).sub.b (C.sub.p H.sub.2p O).sub.c H PA1 R is H or CH.sub.3 PA1 n, p are the same or different and in each case are 2 or 3, with the proviso that n or p has a value of not more than 2.3 in the average molecule, PA1 m =a whole number from 12 to 18 PA1 a =10 to 100 PA1 b =2 to 10, PA1 c =0 to 10 and PA1 10 to 50% by weight of acrylic or methacrylic acid, PA1 25 to 75% by weight of a nonionic vinyl monomer and PA1 2 to 50% by weight of a macromonomer of the formula EQU CH.sub.2 =CR--CH.sub.2 --O--(C.sub.n H.sub.2n O).sub.a (C.sub.m H.sub.2m O).sub.b (C.sub.p H.sub.2p O).sub.c H PA1 wherein PA1 R is H or CH.sub.3 PA1 n and p are the same or different and in each case are 2 or 3, with the proviso that n and p have a value of not more than 2.3 in the average molecule, PA1 m =a whole number from 10 to 18 PA1 a =10 to 100, PA1 b =2 to 10, PA1 c =0 to 10, and
Usually, ionic monomers a) are vinyl compounds, particularly acrylic or methacrylic acid derivatives. In the case of the free acid, its share of component a) is 50 to 100%. The nonionic monomer b) contains a vinyl group and, as a rule, is an acrylate or styrene derivative. Preferably, 50 to 100% by weight of alkyl methacrylate is used as component b). The addition of the cross linking monomer d) is optional.
Of particular importance is the ether c) because, through the suitable selection of the values of n, m, p and R, it is possible to influence the properties of the product selectively. At the same time, an additional enhancement of the thickening effect is ascribed to the hydrophobic R group. As a rule, the ether contains a polyoxyethylene chain B.sub.n between the allyl group and the hydrophobic R group. Additional polyoxypropylene or polyoxybutylene groups, A.sub.m or A.sub.p, can also be present in the molecule. The hydrophobic R group contains at least 8 carbon atoms. It may consist of a polyoxyalkylene chain of propylene oxide or butylene oxide; however, a long-chain hydrocarbon group is preferred. Accordingly, only the use of hydrocarbon groups is disclosed in the examples given in EP-A1 0 216 479.
In particular, hydrocarbon groups with 8 to 30 carbon atoms, such as octyl, lauryl or stearyl, aralkyl, such as 2-phenylethyl, aryl such as naphthyl, alkaryl, such as alkylphenyl or cycloalkyl groups are named.
It is pointed out particularly that the substituents on these groups must not under any circumstances adversely affect the hydrophobic character of the R group, otherwise the additional thickening action, which is brought about by the hydrophobic character of the R group, is lost.
Particularly preferred copolymers are those, which consist of 20 to 60% by weight of acrylic acid and/or methacrylic acid, 5 to 60% by weight of ethyl acrylate or other alkyl (meth)acrylates and 2 to 50% by weight of the allyl ether.
Moreover, copolymers are preferred which consist of 20 to 60% by weight of dialkylaminoalkyl (meth)acrylate or acrylamide, 5 to 60% by weight of ethyl acrylate or methyl methacrylate or other alkyl methacrylates and 2 to 50% by weight of the allyl ether.
The polymerization of components a), b), c) and d) yields an emulsion, in which the thickening agent is present in an insoluble and unswollen state. Only by the addition of alkali or acid is the desired thickening of the emulsion then achieved.
These copolymers can attain molecular weights far in excess of 0.5 million; as a rule, the molecular weights are between 1 and 5 million. Chain transfer agents are therefore added in order to limit the molecular weight to a range from 100,000 to 200,000, in which the desired properties are optimal.
The polymerization is carried out in an emulsion at a pH, at which the polymer is insoluble or is present in the unswollen state. In this way, the polymer can be isolated, for example, by removing it from the emulsion and drying it.
Frequently, however, the use of an aqueous dispersion of the polymer is advantageous. The emulsion can then be added to the solution that is to be thickened; the pH is adjusted to the value necessary to achieve the desired thickening effect by the addition of alkali, either before or after addition of the emulsion to the preparation, depending on the ionic monomer used.
The amount of thickener used is usually 0.05 to 5% by weight of the water content of the preparation that is to be thickened.
The copolymers ar of particular importance for the thickening of aqueous media which contain electrolytes or a dispersed phase. In particular, they are used to thicken paints, printing pastes for textiles, bleaching agents, alkaline liquors or paint removal preparations. Further important applications relate to the production of petroleum or of ores. In addition, they are used as filtration aids or flocculants.
However it has been ascertained that the thickening effects achieved are not adequate for all applications. Frequently, even better thickening effects are desired. At the same time however, the amount of the thickening agent added should remain as small as possible.